This invention in general relates to the art of PBX interface circuitry and more particularly to PBX interface circuitry used for station carrier systems.
With the advent of increased demands on telephone company cable plant facilities, station carrier systems were developed to allow the use of a central office cable pair by more than one subscriber. Generally the systems consist of a plurality of channels, each of which corresponds to a unique subscriber, that are multiplexed on a frequency divided or other basis. One transmission cable pair, therefore, is able to provide communication facilities for a pluraltiy of subscribers and to increase the capibility needed by overloaded central office cable plants.
The usual number of channels provided by such systems vary; however, a common number is six channels with two carrier signals being used for each channel to provide directional discrimination. In general the outgoing carrier signals, which are used to transmit signals from the central office to the subscribers, are continuously and constantly on the line. The inbound carrier signals, which are used to transmit from the respective subscribers to the central office, appear on the line only when the respective subscribers are off-hook and are discontinued when the subscribers are on-hook. The carrier channel equipment at the central office detects off-hook conditions at the subscriber stations by sensing the appearance of the respective inbound carrier signals.
For seizure of the subscriber sets ringing is achieved by superimposing a ringing signal at relatively low energy levels on the carrier signals. The low level ringing signals are demodulated at the subscriber terminal and amplified to the level necessary to actuate the ringer and the subset. This method of initiating incoming and outgoing calls over station carrier systems is incompatible with PBX equipment using ground start trunks from the central office.
Such PBX equipment is usually connected directly to the central office ground start trunk to permit a ground on the tip lead to be sensed at the PBX as an incoming call from the central office and a ground on the ring lead to be sensed at the central office as an outgoing call from the PBX. In addition, for calls initiated from the PBX a battery reversal by the central office indicating a termination must be provided to the PBX for message metering. Station carrier systems generally do not provide optimal facilities for such connections.
It would be advantageous to provide an interface between a plurality of ground start trunks and their associated PBX subscribers using the additional capabilities of station carrier systems to permit the use of one transmission cable pair for voice communications and supervision of the subscriber group.
Accordingly, it is an object of the invention to provide ground start capability to a station carrier system having a plurality of ground start PBX subscribers.